1. Field of the Invention
The present invention relates to an image pickup apparatus for reading out an image which was photoelectrically converted during only an arbitrary period of time.
2. Related Background Art
Hitherto, an electronic still camera or a video camera has been known as an apparatus for controlling photoelectric conversion time of an image pickup element to an arbitrary period of time and for reading out signal charges corresponding to an optical image.
To adjust the photoelectric conversion time in the electronic still camera or video camera, there is used a mechanical shutter in a conventional silver halide still camera or what is called an electronic shutter to control a period of time from the completion of the clearing operation of the signal in the image pickup element to the start of the reading operation. A photographer can set a shutter speed to a desired speed. Or, the photoelectric conversion time is controlled so as always to obtain the optimum light amount in accordance with a lens aperture ratio on the basis of the result of calculation by a microcomputer in the camera.
The image pickup element which is used in such a conventional electronic still camera has a problem that in the case of a photographing operation accompanied by a dark current during a long time (for instance, 1/30 to 1/8 second), the S/N ratio deteriorates or the black level fluctuates in proportion to the photoelectric conversion period of time, so that it becomes very hard to see a still image.